Karl Smith, a professor of public economics and government at the University of North Carolina-Chapel Hill, has a good rule of thumb for categorizing epidemics: If it spreads along lines of communication, he says, the cause is information. Think Bieber Fever. If it travels along major transportation routes, the cause is microbial. Think influenza. If it spreads out like a fan, the cause is an insect. Think malaria. But if it's everywhere, all at once—as both the rise of crime in the '60s and '70s and the fall of crime in the '90s seemed to be—the cause is a molecule.

A molecule? That sounds crazy. What molecule could be responsible for a steep and sudden decline in violent crime?

Well, here's one possibility: Pb(CH2CH3)4.

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In 2007 he published a new paper looking at crime trends around the world (PDF). This way, he could make sure the close match he'd found between the lead curve and the crime curve wasn't just a coincidence. Sure, maybe the real culprit in the United States was something else happening at the exact same time, but what are the odds of that same something happening at several different times in several different countries?

During the '70s and '80s, the introduction of the catalytic converter, combined with increasingly stringent Environmental Protection Agency rules, steadily reduced the amount of leaded gasoline used in America, but Reyes discovered that this reduction wasn't uniform. In fact, use of leaded gasoline varied widely among states, and this gave Reyes the opening she needed. If childhood lead exposure really did produce criminal behavior in adults, you'd expect that in states where consumption of leaded gasoline declined slowly, crime would decline slowly too. Conversely, in states where it declined quickly, crime would decline quickly. And that's exactly what she found.

Nevin collected lead data and crime data for Australia and found a close match. Ditto for Canada. And Great Britain and Finland and France and Italy and New Zealand and West Germany. Every time, the two curves fit each other astonishingly well. When I spoke to Nevin about this, I asked him if he had ever found a country that didn't fit the theory. "No," he replied. "Not one."

We now have studies at the international level, the national level, the state level, the city level, and even the individual level. Groups of children have been followed from the womb to adulthood, and higher childhood blood lead levels are consistently associated with higher adult arrest rates for violent crimes. All of these studies tell the same story: Gasoline lead is responsible for a good share of the rise and fall of violent crime over the past half century.

Like many good theories, the gasoline lead hypothesis helps explain some things we might not have realized even needed explaining. For example, murder rates have always been higher in big cities than in towns and small cities. We're so used to this that it seems unsurprising, but Nevin points out that it might actually have a surprising explanation—because big cities have lots of cars in a small area, they also had high densities of atmospheric lead during the postwar era. But as lead levels in gasoline decreased, the differences between big and small cities largely went away. And guess what? The difference in murder rates went away too. Today, homicide rates are similar in cities of all sizes. It may be that violent crime isn't an inevitable consequence of being a big city after all.

The gasoline lead story has another virtue too: It's the only hypothesis that persuasively explains both the rise of crime in the '60s and '70s and its fall beginning in the '90s. Two other theories—the baby boom demographic bulge and the drug explosion of the '60s—at least have the potential to explain both, but neither one fully fits the known data. Only gasoline lead, with its dramatic rise and fall following World War II, can explain the equally dramatic rise and fall in violent crime.

I can't wait for lead reduction to become a politically charged issue. If this is true, can you imagine how EASY it will be to reduce crime in inner cities? By easy, I of course mean practically and fiscally, not politically.

Interesting and bizarre, but a good read. Who would have thought that inadvertently/unknowingly ingesting all of these chemical toxins into our bodies would alter our state of being... I thought it was negro genetics, and their penchant for sniffing lead, murder and such, which is apparently the source of "black" crime. Retarded.

Stick with the "drugs" that grow naturally upon our earth, if you're going to partake.

Once upon a time there was an article reporting that just maybe Nero and Caligula's courts weren't entirely immoral. Maybe some of the horrors attributed to their time were not entirely the result of moral failures, but to the lead water pipes the Romans used.

Although I'm not 100% convinced yet of the effect of lead on human behavior, I think it is pretty well established that the addition of chemicals to human blood can produce some fairly bizarre behaviors. Anyone notice an alteration of behavior on a Friday evening after drinking alcohol?

Both the Iroquois and the Sauk excused violent behavior, saying that X did not kill Y, alcohol did. The Korean legal system seems to follow that kind of thinking.

There are those who think sugar has a detrimental effect on behavior, most notably with young people.

As time goes on and science better understands the effect of various chemicals on human behavior, it will be interesting to see how the legal system works out the concept of responsibility.

Once upon a time there was an article reporting that just maybe Nero and Caligula's courts weren't entirely immoral. Maybe some of the horrors attributed to their time were not entirely the result of moral failures, but to the lead water pipes the Romans used.

I remember reading that the high level of calcium carbonate in the Roman water supply seemingly resulted in a layer of it accreting in the pipes and insulating the water from the lead. I am not sure how convinced I am by that, though.

In any case, I have been hearing a lot about lead lately, so I am sure the wonders of "clean lead" are going to be a big new Republican talking point.

But having no choice but to breathe in lead in urban areas is a relatively modern phenomenon, measurable & mappable. This hypothesis has been around for a while & the evidence seems at least worthy of consideration. Nothing actionable will happen though -- big oil is too powerful.

OTOH, I handled pure lead on a near-daily basis for more than 25 years working in stained glass studios. Hands constantly blackened with it, breathing lead dust & solder vapors, etc, & undoubtedly dragging substantial traces home where I raised two kids. Two bright & healthy kids, as it turns out, & I'm one of the least violent people I know!

But having no choice but to breathe in lead in urban areas is a relatively modern phenomenon, measurable & mappable. This hypothesis has been around for a while & the evidence seems at least worthy of consideration. Nothing actionable will happen though -- big oil is too powerful.

OTOH, I handled pure lead on a near-daily basis for more than 25 years building stained glass windows & lamps. Hands constantly blackened with it, breathing lead dust & solder vapors, etc, & undoubtedly dragging traces home where I raised two kids. Two bright & healthy kids, as it turns out, & I'm one of the least violent people I know!

So I dont know.

But the kids are in prison now right? Interesting how you left that part out!

Kidding. Seriously though, there could be other factors too. For one, you were already an adult, so perhaps the effects of the lead were less significant for you than a child. As for your children, you might have contaminated their environment to some extent, but perhaps the rest of their home environment was relatively lead-free? Perhaps you raised them in a rural area? Lastly, I don't think anyone would argue that lead (or anything else) causes EVERYONE to become violent and more aggressive. If that were the case, I think crime would have REALLY gotten out of hand in the 70s and 80s.

And yes, such a large presence of lead in our environment, as the article notes, was new at the time and thankfully has been at least partially remedied. 12ax7, did you even read the article? Your comment would indicate you haven't.

But having no choice but to breathe in lead in urban areas is a relatively modern phenomenon, measurable & mappable. This hypothesis has been around for a while & the evidence seems at least worthy of consideration. Nothing actionable will happen though -- big oil is too powerful.

OTOH, I handled pure lead on a near-daily basis for more than 25 years working in stained glass studios. Hands constantly blackened with it, breathing lead dust & solder vapors, etc, & undoubtedly dragging substantial traces home where I raised two kids. Two bright & healthy kids, as it turns out, & I'm one of the least violent people I know!

So I dont know.

I grew up in an area where there was a smelter. A friend of mine did some studies and found that the soil is contaminated with lead and cadmium. Turns out the company and the government had been covering it up for years.

Crime? Except for the heavy metals contamination, it's one of the safest places one can live.